1. Field of The Invention
The present invention relates to an exposure apparatus and, more particularly, to an exposure apparatus for use in a photolithographic process as one step in the manufacture of semiconductor elements, liquid crystal displays, and the like.
2. Description of the Related Art
A photolithographic process for manufacturing semiconductor elements, liquid crystal display elements, thin film magnetic heads, or the like, comprises subjecting a photomask or a pattern formed on a reticle (hereinafter referred to as "mask") to projection-exposure onto a photo-sensitizable substrate, such as a wafer or a glass plate (hereinafter referred to as "substrate"), each with a photosensitizer such as a photoresist, coated thereon.
As an apparatus for conducting such projection-exposure, there has been extensively used an exposure apparatus of a so-called, step-and-repeat exposure system, which comprises exposing a pattern formed on a mask to a given area of a substrate, stepping the substrate by a given distance, re-exposing a pattern formed on the mask, and repeating a series of these steps. As an exposure apparatus of another type, there is known an exposure apparatus of a slit scanning type which comprises exposing a pattern on a mask to a substrate while scanning the mask and the substrate in synchronization relative to a rectangular or arc-shaped illumination area.
In the manufacture of recent semiconductor devices, demands have been made to form a pattern on both front and back surfaces of a substrate while ensuring a high degree of accuracy in relative positions of patterns of both the front and back surfaces thereof, in order to effectively utilize both surfaces. In order to enable the formation of patterns on both surfaces of the substrate, a photo-lithographic process requires exposure of two mask patterns on the front and back surfaces thereof while aligning the mask patterns with each other.
Conventional exposure apparatuses, whether exposure apparatuses of a step-and-repeat system or of a slit scanning type, however, cannot satisfy such demands because they are designed to subject a mask pattern to projection-exposure on only one side of a substrate.
In order to solve problems and difficulties encountered with conventional exposure apparatuses, extensive review and studies have been made to meet the demands for a more sophisticated exposure apparatus and, as a result, an exposure apparatus needs to be developed which can subject a front mask pattern and a back mask pattern to simultaneous projection-exposure onto a front surface and a back surface of a substrate, respectively, while ensuring a high degree of accuracy.